Digital information is often transmitted via network, microwave or satellite in a form that can be captured by anyone having the proper receiving equipment. Compact disks (CD's) and Digital Audio Tape (DAT) provide a single standard digital recording medium for data, software, images and audio. Proposed Multimedia Compact Disks, Digital Video Disks, Super Density Disks or other extensions of compact disk technology will provide higher capacity and bandwidth, enabling digital recording of video. For transmitted or recorded digital information, the ability to make an exact copy is often an essential attribute, enabling exchange, distribution and archiving of information. Sometimes, however, there is a need to prevent copying. For example, it is illegal to make an unauthorized copy of copyrighted material. Software, music and video providers have a need for distribution of copyrighted works in a digital form while preventing unauthorized copying of those works. There is a need for a method of selectively inhibiting copying of digital information. In the following application, the word "transmitting" is intended to include sending data to (and retrieving data from) a recording device. In general, recording will be used for illustration but the concepts apply equally well to other types of digital data transmission.
Digital data is rarely transmitted or recorded in its original digital form. Instead, high density digital transmission or recording typically involves numerous tradeoffs of various constraints, resulting in the original data being encoded into bit patterns that satisfy the various constraints. First, there is typically a tradeoff between recording density and error rate. A finite error rate imposes a requirement for additional information to be added for error detection and correction.
A second constraint deals with the highest permissible transition frequency. In magnetics, a related limitation is commonly called intersymbol interference. Typically, in the recording medium, each transition of state has some distorting effect on neighboring transitions. This distortion imposes a maximum on the number of consecutive transitions at a specified minimum spacing. Alternatively, in any recording medium, there is some maximum rate at which some physical phenomena can switch states.
A third typical constraint is self-clocking. For serial binary data, a clock signal for decoding the data often must be extracted from the timing of the transitions of a read signal (reversal of voltage or current, change of frequency or phase, change of light intensity, etc.). There must be an adequate frequency of transitions to keep the clock signal synchronized.
Serial binary data is often physically in a format called NonReturn to Zero Inverted (NRZI). In NRZI format, the waveform is at one state until a binary one occurs, at which time the waveform switches to an opposite state. The maximum transition rate or intersymbol interference limitations discussed above impose a minimum on the amount of time that can pass between transitions. The requirement for self clocking imposes a maximum on the amount of time that can pass with no transition. A code that satisfies the maximum transition rate constraint, the self-clocking constraint, and the NRZI format requirements is commonly called a Runlength Limited (RLL) code. In a RLL code, the number of consecutive binary zeros in the encoded bit pattern must be at least as large as a specified non-zero minimum and no greater than a specified maximum. For example, compact disks typically use a code specified as (2,10)-RLL which means that the number of consecutive zeros in the encoded bit pattern must be at least 2 and no greater than 10.
A fourth typical constraint on the encoded binary signal is a requirement for a limit on the low frequency content of the read signal. In many read channel detectors, a transition is indicated when the read signal crosses a fixed threshold. Any low frequency content in the read signal can cause an offset, restricting the dynamic range of the detector. In addition, for writeable optical disks, track following and focusing signals may be implemented using low frequency modulation of the read signal. Any low frequency content in the read signal may interfere with track following and focusing. Referring again to NRZI format, assign the value +1 to one state of a signal and assign the value -1 to the opposite state. A sum of these values (or the area under the curve) is called Digital Sum Variance (DSV) or alternatively Running Digital Sum (RDS). For many detectors, there is a specified maximum DSV or RDS, and any DSV exceeding the specified maximum is likely to cause data read errors or servo problems.
It is common to encode the original digital data into other digital data that satisfies the above constraints. Typically, the original data is divided into symbols, where a symbol is a small fixed number of bits, typically one byte (8 bits). Typically, each symbol is used as an index into a look-up table containing bit patterns (called channel bits) that satisfy the various constraints. For example, for compact disks, the current standard format divides the original data into 8-bit symbols, each 8-bit symbol being used as an index into a table of channel bit patterns, each channel bit pattern in the table having 14 bits. The corresponding encoder is commonly called an EFM encoder, for "eight-to-fourteen modulation." Each of the 14-bit patterns satisfies the (2,10)-RLL constraint described above. However, when one 14-bit pattern is concatenated with another, the combination of the end of one pattern and the beginning of another pattern may cause a violation of the (2,10)-RLL constraint and some strings of concatenated patterns may violate a DSV constraint. An additional 3 bits, called merge bits, are inserted between the 14-bit table patterns to merge the end of one pattern to the beginning of the next pattern. With appropriately selected merge bits, the resulting channel bits satisfy the (2,10)-RLL requirement and satisfy the DSV requirement. The net result is that for every 8 bits of un-encoded data, 17 bits are recorded (not fourteen as suggested by the name EFM).
For multimedia recording, an improvement to EFM encoding has been proposed, called EFMPlus. In EFMPlus, the encoder is a state machine. For each state of the state machine, there is a separate look-up table. In general, for each symbol, the corresponding channel bit pattern varies depending on the state. Note, however, that the channel bit pattern may be the same for at least two states. In addition, each table entry also specifies the next state. EFMPlus eliminates the merge bits, and instead uses slightly longer table entries and sophisticated substitution of alternative channel bit patterns. The encoder typically looks ahead to oncoming symbols and their possible alternative patterns, choosing the most appropriate pattern for both RLL and DSV. The net result of the EFMPlus proposal is that for every 8 bits of unencoded data, 16 bits are recorded, for a density improvement of about 6% relative to EFM. In the proposed standard for EFMPlus encoding, the look-up table entries are standard to enable unambiguous decoding. However, the encoder algorithms for deciding which alternative bit pattern and corresponding next state will typically vary from manufacturer to manufacturer. Therefore, different manufacturers may encode identical symbol sets into different binary channel bit sequences for recording, but all such sequences can be unambiguously decoded. For additional general information on Multimedia Compact Disks and EFMPlus, see for example, K. A. S. Immink, "EFMPlus: THE CODING FORMAT OF THE MULTIMEDIA COMPACT DISC," IEEE Transactions on Consumer Electronics, Vol. 41, No. 3, pp 491-497, August 1995.
Neither EFM or EFMPlus can completely guarantee satisfaction of the DSV constraint. In each encoding method, there are possible sequences of channel bits that can result in a large accumulated DSV. In EFMPlus, the encoder system designer can reduce the probability of a large accumulated DSV to some commercially acceptable probability but cannot reduce the probability to zero. Even though all permissible sequences of channel bits can be unambiguously decoded from a digital point of view, there are potential analog dynamic range problems in the read channel that may prevent proper decoding or that may interfere with tracking in writeable versions. There are opportunities to exploit this attribute to inhibit copying.
There are additional formatting details for proposed multimedia recording standards that create additional opportunities for copy protection. In proposed Digital Video Disk standards, original digital information is divided into blocks, each block including row and column error correction. There are opportunities to exploit various properties of block formatting to inhibit copying.